Problem: Which of these numbers is prime? ${27,\ 55,\ 59,\ 63,\ 72}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 27 are 1, 3, 9, and 27. The factors of 55 are 1, 5, 11, and 55. The factors of 59 are 1 and 59. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 72 are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, and 72. Thus, 59 is a prime number.